Market demands continue to force improvements in the capabilities and features of modern, state-of-the-art semiconductor circuits and systems. These improvements drive a race towards ever-decreasing feature sizes, the minimum dimensions of the components which make up circuits, rendering the components increasingly difficult to construct. Modern integrated circuits, commonly called chips, are widely used in a great variety of devices and systems, thus construction of the chips must be both efficient and cost effective. Chips are typically manufactured using highly complex techniques based on elaborate, multi-step processes including lithographic (printing) processing steps and chemical (developing) processing steps. The processing is performed on a semiconductor substrate such as silicon, although other materials, including compound semiconductors formed from materials such as gallium and arsenic are also used. Modern fabrication processes may consist of hundreds of manufacturing and handling steps. In order for a given electronic device or system to function as designed, each and every step in the fabrication process is required and must be completed successfully. Each lithographic, processing, and handling step has a critical and unique purpose. For example, the lithographic process steps are used to expose, or “print,” desired patterns and features onto a semiconductor substrate. The process of printing a semiconductor substrate is reminiscent of photographic processes involving shining light through a negative to expose light sensitive paper. In the case of semiconductor printing, specific wavelengths of light are shone through masks to print extremely fine structures onto a substrate. Following a printing step, the physical structures are revealed by chemically processing the printed substrates. Returning to the photography analogy, the chemical processing “develops” the fine structures and patterns on the substrates much as certain chemicals develop images on light-exposed photographic paper. At a specific level on the substrate, the chemical processing removes unwanted, superfluous material while leaving intact the desired structures and patterns printed during the lithographic step. The lithographic and chemical processing steps are repeated as many times as required to produce the desired configuration on the substrate.
While other lithography techniques exist, photolithography remains the predominant technology used today for the printing of minimum-sized features and structures onto a semiconductor substrate. In these light-based approaches, light shines through a mask to transfer the mask pattern onto the substrate. The printing may be positive (direct transfer) or negative (reversed transfer). Since the minimum feature sizes of patterns and structures on the chips are now comparable to, smaller, or even much smaller than the wavelength of visible light, lithographic techniques must constantly evolve to keep pace. Improved mask techniques have been developed, as have light sources with shorter wavelengths to support printing of ever-smaller features and structures. The combination of improved masks and decreased light wavelengths has significantly improved the resolution of fine feature and structure sizes on a chip. After the features and structures have been printed, advanced chemical processing must resolve the small design details in such a way that all of the resulting features and structures operate correctly and as designed. In order to produce a working chip, the many lithographic and chemical processing steps must all function properly to accurately produce the desired patterns and structures which make up the chip.